1. Field of the Invention
The present invention broadly relates to telecommunications. More particularly, the present invention relates to the termination of OAM cells in an ATM network.
2. State of the Art
Perhaps the most awaited, and now fastest growing technology in the telecommunication field in the 1990's is known as Asynchronous Transfer Mode (ATM) technology. ATM is providing a mechanism for removing performance limitations of local area networks (LANs) and wide area networks (WANs) and providing data transfers at a speed of on the order of terabits/second. The variable length packets of LAN and WAN data are being replaced with ATM cells which are relatively short, fixed length packets. Because ATM cells can carry voice, video and data across a single backbone network, the ATM technology provides a unitary mechanism for high speed end-to-end telecommunications traffic.
In order for ATM technology to develop, it must be functional in its own right as well as compatible with existing technology. To be compatible, on the one hand, the ATM cells must be capable of receiving and accommodating voice, video, and LAN and WAN type data; while on the other hand, ATM cells must be capable of adapting to high speed technology such as the synchronous optical network (SONET). In order to meet these and other requirements, a technical group called the ATM Forum which is comprised of numerous corporate representatives has been proposing ATM "standards" which are being provided to ANSI and the ITU-T for their consideration and adoption. Details of those standards may be found in proceedings of the ATM Forum. Of particular interest among the ATM Forum, ANSI and ITU-T documents is a document which is incorporated by reference in its entirety herein and which is dated November 1995 and entitled "B-ISDN Operation and Maintenance Principles and Functions, ITU-T Recommendation I.610".
As is set forth in Section 2 of Draft Recommendation I.610, Operation, Administration, and Management (OAM--also called Operation and Management) cells provide performance monitoring, defect and failure detection, system protection, defect information, and fault localization functions. Performance monitoring is a function which processes user information to produce maintenance information specific to the user information. The maintenance information is added to the user information at the source of a connection/link and extracted at the sink of a connection/link. Analysis of the maintenance event information at the connection sink allows analysis of the transport integrity. Defect and failure detection is accomplished by continuous or periodic checking and results in the production of various alarms. In the system protection function, the effect of a defect on the transport of user information is minimized by blocking or changeover to other entities, and the failed entity is excluded from operation in order to protect the system. Response to status report requests will also be given. Fault localization involves a determination by internal or external test systems of a failed entity if defect information is insufficient.
As set forth in Section 7.1 of the ITU-T Recommendation, and as seen in prior art FIG. 1, the ATM layer OAM cell includes five bytes of header followed by a forty-eight byte OAM Cell Information Field. Details of the five bytes of header include are set forth in Recommendation I.361 which is hereby incorporated by reference herein in its entirety. Recommendation I.361 states that for F4 virtual path connection flow identification (belonging to the ATM Layer Management), two pre-assigned VCIs are used to distinguish OAM cells meant for VPC (Virtual Path Connection) end-to-end or VPC segments. These two values are defined in Recommendation I.361. For F5 virtual channel connection flow identification (also belonging to the ATM Layer Management), two PTI values are used to distinguish OAM cells for VCC (Virtual Circuit Connection) end-to-end or VCC segments. These two values are defined in Recommendation I.361. Since OAM functionality is on a per VP or VC basis, the five byte headers of the OAM cells contain the VPI (for F4) or VPI/VCI (for F5) of the connection with which the OAM cell is associated.
Turning to the forty-eight byte OAM Cell Information Field seen in FIG. 1, the first four bits define the OAM cell type; e.g., fault management, performance management, and activation/deactivation. The next four bits identify the OAM function type; i.e., the type of function performed by the cell within the management type. For fault management, the function types include an alarm indication signal (AIS), a remote defect indication (RDI), a loopback, and a continuity check. For performance management, the function types include forward monitoring, and backward reporting. For activation/deactivation, the function types include performance monitoring, and continuity check. Following the four bit OAM function type field, forty-five bytes of information pertinent to the function type are sent, followed by a six bit reserved field for future use, and a ten bit error detection code field.
A desirable mechanism for fault localization, pre-service connectivity verification, and on-demand connectivity monitoring is the use of an OAM loopback cell (which, as set forth above, is a fault management function). The ATM layer loopback capability allows for OAM loopback cells to be injected at one location, along a VPC or VCC, and returned at a different location, without having to take the connection out of service. Thus, a loopback cell can be injected at any accessible point along the VPC or VCC (i.e., at either end-point or at any connecting point). This cell is looped back at a downstream segment end-point or connection end-point specified by the information contained in the OAM cell. Details of the loopback capability are described in previously incorporated U.S. Ser. No. 08/573,244, now U.S. Pat. No. 5,659,540.
While the loopback and other OAM capabilities provide great advantages in the management of ATM networks, it will be appreciated that a major limitation of managing an ATM network is the possibility that an end-point does not support OAM functionality. In particular, the connection end-point may be a router, frame relay equipment, a LAN, or other non-OAM-supporting equipment. In such a situation, OAM cells which are received by the non-OAM-supporting end-point will either be discarded, ignored, or improperly interpreted by the receiving equipment. In any event, the desired result of system management will not be attained as the OAM cells will not be properly terminated.